The invention relates to apparatus and a method for investigating the properties of a solid material by inverse chromatography, and in particular by inverse gas chromatography.
In conventional gas or liquid chromatography, the composition of an unknown multicomponent gas or liquid is determined by injecting the unknown material into a carrier fluid (the xe2x80x9cmobile phasexe2x80x9d, typically helium or nitrogen) which passes through an absorptive column (the xe2x80x9cstationary phasexe2x80x9d). The retention time due to mobile-stationary phase interactions (solid-vapour, vapour-liquid or solid-liquid adsorption) of the various eluted components allows for the composition of the unknown material to be deduced by reference to calibration data. In contrast, in inverse gas or liquid chromatography the properties of an unknown solid material in a column are investigated by passing a known probe through the column. For inverse gas chromatography, the carrier again is typically an inert gas such as helium or nitrogen, and the probe may be a material which is a gas under ambient conditions, or a solvent vapour.
The length of time that the probe is retained in the column is determined by the physicochemical interactions between the probe and the solid material packed in the column. This interaction is governed by a number of mechanisms, the most important of which are surface adsorption, and where the probe is partially soluble in the solid material, bulk solubilisation. Thus, the study of the retention behaviour of a probe can provide important information on the surface and bulk properties of the material in the column.
The use of inverse gas chromatography in investigating various properties of solids, such as adsorption isotherms, adsorption thermodynamics, surface energetics, acid-base interactions, glass transition temperatures, surface area, diffusion coefficients and porosity is described in the book xe2x80x9cPhysicochemical Measurement by Gas Chromatographyxe2x80x9d by J. R. Conder and C. L. Young., John Wiley and Son, 1979 and the article xe2x80x9cDaryl Williams, Inverse Gas Chromatography, Characterisation of Composite Materials: Materials Characterisation Seriesxe2x80x94Surfaces, Interfaces, Thin Films, Chapter 5, 80-103 Butterworth-Heinemann, 1994. This article also discusses in detail the most appropriate experimental method, for example xe2x80x9celution at infinite dilutionxe2x80x9d, xe2x80x9celution of a characteristic pointxe2x80x9d or xe2x80x9cfrontal analysisxe2x80x9d, for determining each of these properties. In addition to influencing the method chosen, the nature of the property under examination will also affect the choice of probe.
In most instruments used for inverse gas chromatography, the probe is a single compound which is introduced into a single component carrier gas flow that is then eluted through the column. For example, J. Chrom. A. 715, 1995, 279-285 discloses an injection system for an inverse gas chromatograph composed of a closed loop vapourisation chamber with a gas sampling valve, which provides multiple injections of the same sample into a single component gas flow at precisely controlled intervals.
A further variation is disclosed in U.S. Pat. No. 4,806,315 (Daigle). This reference is concerned with the ability to measure samples containing water more accurately by adding water vapour to the carrier gas flow, to form in effect, a two component carrier gas flow (e.g., water and helium). However, in the method disclosed, water passes into the flow via a water permeable membrane, and therefore the ratio of the two components (i.e., the humidity of the carrier) is fixed by the physical properties of the membrane. The only way to vary the humidity of the carrier is to dismantle the equipment and replace the membrane with one of a different type and size.
U.S. Pat. No. 4,869,094 (Gilber) relates to a method of determining sorption isotherms of food by inverse gas chromatography. In the method disclosed, a temporary front of water is established in a carrier gas which can be regarded as a two component gas flow composed of water and carrier gas. Although this reference discloses the possibility of investigating the behaviour of a solid material through inverse gas chromatography, under conditions of humidity, it does not disclose any method by which the humidity level can be varied. The two component carrier flow is formed by passing helium through a water trap, and thus the humidity of the flow is fixed at 100%. The only way therefore of changing the concentration of water vapour in the gas flow would be to change the temperature of the water bath surrounding the pre-column and the experimental column. This would, of course, necessitate a change in the temperature of the sample in the column, thereby influencing the behaviour of the material in the column since adsorption phenomena are highly temperature dependant.
According to the present invention there is provided apparatus for investigating the properties of a solid material comprising:
means for generating a flow of a carrier fluid, comprising at least two fluid components;
means for generating a second fluid flow comprising the said carrier fluid components and probe material;
a column, for holding the solid material;
means for selectively passing the flow of carrier fluid and the second fluid flow through the column;
means for controlling the temperature of the column;
a detector for detecting the passage of the probe material through the column, and
means for controlling the relative proportions of the said fluid components in the flow of carrier fluid and the second fluid flow.
Preferably, the carrier fluid is a gas. The probe material may be a gas under normal conditions (e.g. methane) or may be a vapour of a substance normally liquid under normal conditions (e.g. octane).
In a particular preferred embodiment of the invention, one of the said carrier fluid components is water i.e. the means for controlling the relative proportions of the carrier fluid components comprises means for controlling the relative humidity of the carrier and second fluid flows. The ability to vary the humidity of the carrier enables the solid material to be investigated under a wide range of conditions and enables the simulation of conditions which may arise in use. This is particularly important where for example, the solid material is a compound intended for pharmaceutical use.
In another embodiment of the invention the means for generating a flow of carrier fluid comprises:
means for generating a first sub-flow relatively rich in a first said component (for example a stream of an inert gas such as helium);
means for generating a second sub-flow relatively rich in a second said component (for example a saturated stream of the same gas);
means for combining the first and second sub-flows,
wherein the means for controlling the relative proportions of the said fluid components comprises at least one control valve for varying the mixing ratio of the said first and second sub-flows (for example so as to control the humidity of the carrier).
In a further preferred embodiment, a third sub-flow, relatively rich in the probe material is also provided (for example, of humidified helium containing the desired probe material) and mixed with the first and second sub-flows in a desired ratio.
It is therefore possible for variations in the proportions of the carrier components to be made without the need to vary other experimental parameters such as column temperature. Furthermore, since the proportion of components present is not predetermined by the intrinsic properties of the apparatus, as in U.S. Pat. No. 4,806,315, but rather is varied simply by adjusting a control valve, the relative proportions of the fluid components may be varied both before and during an investigation.
Preferably, the second sub-flow comprises both the first and second components (for example, inert gas and water vapour) and the means for generating the second sub-flow comprises a container for the second component in liquid form. The container has an inlet connected to a fluid line, and a vapour space over the liquid, so that a flow of the said first component can be passed through the vapour space, and thereby generate the xe2x80x9csub-flowxe2x80x9d through the said vapour space.
The said inlet will generally be below the liquid level of the second component, so that the first component (the inert gas) bubbles through the second component, on passage through the container.
In a further embodiment of the invention, a second fluid flow (i.e. a probe-containing flow) may be generated in a similar fashion, by bubbling gas through the probe material in liquid form. In this embodiment, the apparatus comprises at least one second container, for containing the probe material in liquid form, the said container having an inlet and an outlet, and means defining a vapour space over the said liquid probe material. A fluid line is connected to the second inlet, for passing a flow of at least one of the carrier flow components through the vapour space.
In this embodiment, the inlet is again preferably below the level of the probe material, so that the first component bubbles through the said probe material, on passage through the container.
By providing more than one such container for different probe material, with appropriate switching arrangements, it becomes possible to switch easily between various types of probe material.
Control valves, which may preferably be operated automatically, under computer control, may be provided for varying the mixing ratio of the said first, second and third sub-flows.
Because the relative proportions of the fluid components in the second fluid flow are varied by altering the mixing ratio of the various sub-flows, any changes to the concentration of components in the fluid flow can be brought about independently of, for example, the temperature of the column.
When the probe is a material which is a gas under normal conditions, the means for generating a second sub flow may comprise simply a fluid line for introducing the probe material, and the means for controlling the relative proportions of the said fluid components may comprise a control valve for varying the mixing ratio of the said probe material and the said two fluid components.
The apparatus may also include an injection port for injecting probe material into the carrier gas flow.
The apparatus in its preferred embodiments permits more than one method of introducing probe material into the carrier gas flow. This allows a number of different types of compounds to be employed as probes.
It is preferable that the apparatus comprises at least two columns since this enables the results from a column packed with the solid material under investigation to be compared with those from an empty column or one containing inert material, and for batch to batch variations and the reproducibility of samples and methods to be examined under identical conditions. A column selector valve may be employed to select the column into which the probe-containing flow is to be passed. A selector valve may also be employed to select the flow which is passed to the column or column selector valve, the selection being made, for example, between a probe-containing flow, and a non-probe-containing flow, the flow which it not selected at any particular time being vented from the apparatus.
In a preferred embodiment, probe-containing flow may be passed though one of the columns, whilst non-probe-containing flow, is passed through another. In this way, one column can be prepared for use while an experiment is being carried out on another column. This is of advantage to an operator since a packed column often needs to be conditioned for several hours or days to allow it to equilibrate before an experiment can be commenced.
The apparatus preferably includes at least three, more preferably, at least 4, and more preferably still, at least 5 temperature zones, and means for controlling independently the temperature in each said zone.
This enables those portions of the apparatus within a particular zone to be set at an optimum operating temperature, rather than a xe2x80x9ccompromisexe2x80x9d temperature under which all parts of the apparatus can function.
In particular, the column and the flow selector valve are preferably in different temperature zones. The column is preferably also in a different temperature zone from the part of the apparatus in which the probe is introduced, particularly when a probe-containing flow sub-flow is generated from a liquid probe material.
The temperature of the zone or zones containing liquid probe material is determined by the physical properties of the fluid components employed, for example its boiling point. However, in order to investigate a wide range of solid material properties, it is desirable to be able to vary the column temperature over a considerably broader range. Thus, the ability to locate the liquid probe material in a different temperature zone from the column provides a considerable advantage.
The column and the injection port are preferably also in different temperature zones.
In a particularly preferred embodiment, the container for the second component (e.g. water) and the container for liquid probe material are in a first temperature zone, the column is in a second temperature zone, and the injection port and selector valves are in a third temperature zone.
It is also advantageous for the various selector valves to be located in different temperature zones from containers for the second component (e.g. water) and for liquid probe material, in order to minimise the risk of condensation of fluid components.
In a further particularly preferred embodiment, the container for the second component (e.g. water) and the container for liquid probe material are in a first temperature zone, the column is in a second temperature zone, the injection port is in a third temperature zone, and the flow selector valves are in a fourth temperature zone.
Where the carrier fluid is a gas, it is an advantage for the injection port to occupy a different temperature zones, since this enables its temperature to be maintained at which allows for the complete vaporisation of any liquid to be introduced through the injection port.
The temperature of the column can preferably be varied from ambient (xe2x88x92298 K) to 100xc2x0 C. (373 K), preferably from 77 to 500 K, and more preferably from 77 to 600 K. The apparatus preferably also comprises means for pre-setting the temperature of at least one of the said temperature zones prior to an investigation of the properties of the said solid material.
The automatic control function of the apparatus may preferably comprise means for pre-programming flow of carrier fluid and the second fluid flow through the column prior to an investigation of the properties of the said solid material, and/or for pre-programming the relative proportions of the said fluid components prior to an investigation of the properties of the said solid material.
The ability to pre-set the experimental conditions allows the apparatus to be programmed to carry out a number of experiments in a row, for example overnight, thus maximising the efficient use of time.
The proportion of each fluid component in the flow of carrier fluid and the second fluid flow can preferably be varied over a range of from 0 to 95 percent, by altering the mixing ratio of the various sub-flows. The concentration of components present may be thus be varied in a step-wise, ramped or modulated manner.
The principal component of the carrier will generally be an inert gas, such as nitrogen, helium or argon, and the probe material will generally comprises at least one organic compound.
In a further preferred embodiment of the invention, the column is straight and is vertically mounted in such a way that the longitudinal axis of the column is vertical.
The use of a straight column allows the column to be packed more easily without gaps or breaks forming during packing. Furthermore, mounting the column vertically prevents gaps or channels developing if the solid material settles during conditioning since any settling will simply shorten the packed length of solid material.
It can therefore be seen that the apparatus is particularly suited for a determining a wide range of properties of a solid material in the column since it is capable of generating a wide range of operating conditions.
Appropriate software may be provided to automate the measurement of, for example, net retention time, net retention volume, specific retention volume, column inlet and outlet pressures, and fluid-solid distribution coefficient, and for determining therefrom properties of the solid material such as heat of adsorption, standard free energy of adsorption, entropy of adsorption, dispersive component of solid surface energy, total uptake per unit of solid material, surface area, thermal desorption, glass transition temperature, phase transition properties, diffusion coefficients, porosity, polymer-polymer interaction parameters, probe-polymer interaction parameters, acid-base surface properties and competitive adsorption.
In another aspect of the invention there is provided, a method of investigating the properties of a solid material comprising:
inserting the solid material into the column of apparatus as described above;
passing the flow of carrier fluid over the solid material;
passing the second fluid flow over the solid material;
detecting the passage of probe material through the column to determine properties of solid material.